I'm trying to figure out what rear differential is in my '72 without counting the teeth. I run 65 mph @ 3100rpm's on the highway. Anyone want to give me an estimate??? The trans is a TH400.

Thanks in advance,
Craig

 

Depends on tire size. I'm guessing you have a 3.55 rearend.

Here's the formula if you don't want to jack up the rearend and count the driveshaft revs per wheel revs.

Rear ratio = (2.98 times tire outer diameter in inches) divided by miles/hour per 1,000 rpm

This should get you close enuf that you can tell what the actual ratio is.

 

Thanks, I'll look at what size they are tonight. I know that they are bigger than stock tires (Goodyear Eagle GT II's) as my speedo says 10mph faster than I'm actually going. I took a handheld gps with me and 76 on the speedo was 75 on the gps.

 

I've got 255-60X15 Eagle GTs and a 3:55. I get 22 mph per 1,000 rpm in 4th.

 

Ooops...Typo in my previous post. It's was actually 65 on the gps and 76 on the speedo. Thanks for your input!

 

Thanks, my tires are 255/60/15's as well. I wonder if the ratio of a turbo400 in third is the same as a four-speed car in 4th?? If they are the same....then my rear is 3:55. [sm=gears.gif]

 

If you come off the line pretty good you probabbly have a 3:55.
If you bog a bit then figure on a 3:36
If you engine is screaming at 55MPH figure on 3:73 or so on.

 

Both transmissions are 1.00 to 1, so yes, you do have 3.55 gears.

 

I would say your close to 3.50

 

Thanks alot!! Does anyone know if the 3.55 gears were standard or an option?